(1) Field of the Invention
The present invention relates to an image formation apparatus comprising a fixing device, and in particular to technology for, when performing thermal fixing on a small-sized recording sheet by using an induction heating method, suppressing an excessive temperature increase in portions of a heat generation member that do not come into contact with the small-sized recording sheet.
(2) Description of Related Art
In recent years, fixing devices that utilize compact heat sources of an induction heating type having a relatively high heat conversion efficiency have been used in some image formation apparatuses of an electrophotographic type, or an electrostatic recording type. Such fixing devices draw great attention due to their abilities to conserve energy, save space, reduce a warm-up period, and so on.
In a case where magnetic flux, which is generated by supplying an alternating electric current to an excitation coil, is guided to a conductive heat generation layer by core members (e.g., ferrite cores) so that a specific part of a heat generation member is heated, the heat generation member can be constructed with a significantly small heat capacity. Use of such a heat generation member reduces the warm-up period to a great extent.
With the above structure, however, only a small area of the heat generation member is in contact with other components, with the result that the heat cannot easily be transferred. In this case, if a print job to print on a recording sheet having a small width (hereinafter, “small-sized sheet”) is repeated continuously, the temperature of portions of the heat generation member that do not come into contact with the recording sheets (hereinafter, “contactless portions”) will be abnormally increased, the contactless portions being outer edges of the heat generation member in the width direction thereof. This may thermally damage or deteriorate components positioned in the vicinity of said contactless portions. Also, in a case where a print job to print on a recording sheet having a large width (hereinafter, “large-sized sheet”) is executed immediately after the aforementioned repetition of the print job to print on the small-sized sheet, several defects such as hot offset and uneven glossiness appear on the outer edges of the large-sized sheet in the width direction thereof.
There are methods to suppress a temperature increase in said contactless portions in the above-described fixing device. Such methods include a technique to shield only said contactless portions from the magnetic flux by moving conductive materials in accordance with a sheet width, and a technique to, with use of a degaussing coil, cancel out a part of the magnetic flux over said contactless portions.
The following documents disclose techniques to suppress an excessive temperature increase in said contactless portions by incorporating a magnetic shunt alloy, whose Curie point is somewhat higher than the fixing temperature, into the heat generation member of the above-described fixing device. With the presence of such a magnetic shunt alloy, the heat generation member has the self-temperature control function—i.e., when the temperature of said contactless portions has been increased to the Curie point, said contactless portions automatically turn nonmagnetic, thus reducing the amount of heat generated in the heat generation member.
Japanese Patent Publication No. 3988251 discloses, for example, an image heating device that causes a heat generation layer, which includes a magnetic shunt alloy sublayer, to come into contact with the inner surface of a fixing belt. The fixing belt in this document has a significantly small heat capacity, rendering a warm-up period extremely short. It is further described in this document that, as the self-temperature control is effectively performed, a magnetization member would not get thermally damaged, even when a print job to print on a small-sized sheet has been repeated continuously.
Japanese Patent Application Publication No. 2007-156065 discloses a fixing device including a shield plate that (i) is positioned inside a cylindrical heat generation roller made of a magnetic shunt alloy, extending along an axial direction thereof, and (ii) has a C-shaped cross section. Here, the shield plate is positioned such that an edge of each end portion of the C-shape is closest to the heat generation roller. This structure makes the thermal load on these end portions small, suppresses an excessive temperature increase, reduces a warm-up period, prevents occurrence of an offset, and provides a high-quality fixing performance.
Japanese Patent Application Publication No. 2007-264421 discloses a fixer including a low-resistance plate member that is positioned inside a cylindrical fixing rotary body, extending along an axial direction thereof. The central portion of the low-resistance plate member in the axial direction of the fixing rotary body is thinner than end portions thereof, each of the end portions having a larger thickness than the thickness by which magnetic flux can penetrate thereinto. According to this document, the self-temperature control function is effectively realized particularly on end portions of the fixing rotary body in the width direction thereof. An excessive temperature increase in these end portions can be suppressed without lowering the warm-up performance and heating efficiency of a central portion of the fixing rotary body. It is described in this document that, even when a print job to print on a small-sized sheet has been repeated continuously, the excessive temperature increase in the end portions of the fixing rotary body can be reliably suppressed without causing under-heating on the central portion of the fixing rotary body.
Meanwhile, during a stand-by period (i.e., while the image formation is not being executed), the temperature of a fixing device needs to be maintained at a stand-by temperature from which the fixing device can reach the fixing temperature within a few seconds, so as to promptly perform the fixing whenever an instruction to execute the image formation is issued. Especially, if the fixing device cannot perform the warm-up promptly, it will be essential that the stand-by temperature is high. This is not suitable for energy conservation. Furthermore, if the fixing device cannot perform the warm-up promptly, the user will have to wait for a while after turning on the power of the image formation apparatus, which is not favorable.
By making the fixing device compact in structure and reducing the heat capacity thereof, the fixing device can effectively conserve energy and promptly execute the warm-up. Accordingly, there are demands for yet more compact fixing devices.
A compact fixing device having high heat generation efficiency and a self-temperature control function may seem to be easily constructed by, for example, using a magnetic shunt alloy layer as a base member for the fixing belt, and heating the base member by electromagnetic induction. However, it is not easy to manufacture a fixing belt having a magnetic shunt alloy layer of a uniform thickness. It is not impossible to manufacture such a fixing belt, but it would be difficult for such a fixing belt to have all the essential properties (e.g., temperature characteristics and strength) it should have. Such a fixing belt could be extremely costly as well.
On the other hand, when a base member for the fixing belt is made of a conductive material that is not a magnetic shunt alloy, it is possible to use, as the base member, a conductive material that is relatively easily manufacturable, has excellent properties, and is inexpensive. Such a conductive material can also be heated by electromagnetic induction. For example, when nickel is chosen for the base member, an electroformed nickel belt would be good to use, because it has been manufactured for a long time and widely used, is relatively easily manufacturable, and has great strength.
However, when a conductive material that is not a magnetic shunt alloy is provided as the base member of the fixing belt to serve as a conductive heat generation layer, it is considered that the stated self-temperature control function is difficult to realize using conventional technologies, unlike a case where a conductive material is provided as one of constituents of the fixing belt for assisting the fixing belt in generating heat.